The present invention relates to a light source for a wavelength division multiplexing optical communication system, and, more particularly, to a multi-wavelength single mode laser using sampled grating and a method for fabricating the same.
Recently, as a scheme for increasing transmission capacity to meet with the tremendous amount of information transferred via communications, optical data transmission by wavelength division multiplexing (WDM) is used.
In the WDM system, optical signals whose wavelengths (channels) are different from each other are transferred through an optical fiber by using non-interferential property of light so that the transmission capacity can be increased. The WDM scheme has an advantage in ensuring expansibility and flexibility of the optical communication system. Especially, in construction of optical subscriber network such as ATM-PON and etc., the WDM method is considered as more economical than a time division multiplexing (TDM) method.
As a light source for such a WDM system, a multi-wavelength distributed feedback (DFB) laser array seems most potent because of its good single mode characteristics (side-mode suppression ratio higher than 30 dB). The multi-wavelength light source may be realized by combining discrete DFB lasers externally. However, it will be more desirable to fabricate the multi-wavelength DFB laser array on a single substrate for economic reasons.
The DFB laser comprises a wavelength selective diffraction grating in its structure. The oscillation wavelength xcex of the laser is determined by the grating period xcex9 of the diffraction grating and an effective refractive index neff of a waveguide as follows:
xcex=2neffxcex9xe2x80x83xe2x80x83Eq.(1)
As shown in Eq.(1), in order to implement the multi-wavelength laser array, there are provided a method for implementing diffraction gratings having different periods from each other on a single substrate and another method for adjusting the effective refractive indexes with the same grating period.
Firstly, the method of implementing diffraction gratings with different periods is considered. There are several ways of grating fabrication for a conventional DFB laser. Usually typical photolithography cannot be used because the grating period is as small as around 240 nm, which is beyond the resolution of it. Therefore, in order to manufacture the diffraction grating, e-beam lithography, holographic interference exposure, and a phase mask are currently in use.
However, they have problems for the application to a multi-wavelength laser array. Among them, the holographic interference exposure scheme is not proper for manufacturing the multi-wavelength light source, because it forms the diffraction gratings having same periods on the full surface of a wafer. The E-beam lithography scheme is expensive and time-consuming so that it is not proper for mass production. The phase mask scheme seems not to easily implement because it is not easy to form the phase mask.
On the other hand, an attempt for fabricating the array of the multi-wavelength DFB laser through the change of effective refractive index is realized by selective area crystal growth. In the selective area growth, dielectric thin film is deposited on a semiconductor substrate and, then, the dielectric thin film is selectively removed to form a mask pattern. At this time, thickness of a grown layer can be controlled by mask width and distance between the masks, which can be used to change the effective refractive index of a laser on the same wafer.
In xe2x80x9cDetuning adjustable multi-wavelength MQW-DFB laser array grown by effective index/quantum well energy control selective area MOVPExe2x80x9d, M. Aoki and et al., IEEE Photonic technology letters, vol.6, No.7, pp.789-791, July 1994, there is shown that the wavelength of the DFB laser that is fabricated by this technique can be changed by around 10 nm.
Also, U.S. Pat. No. 5,784,183 issued to the same author in 1998 shows the wavelength change of 30 nm.
However, if thickness difference increases too much by the selective area crystal growth, problems as follow could happen. As the mask width increases and the space between the masks decreases in order to enhance the thickness difference, optical characteristic of a quantum well deteriorates. Furthermore, InGaAsP composition of a laser active layer varies with the thickness enhancement so that strain is incorporated in the grown layer. When the amount of strain is too high, the laser characteristic is deteriorated because of the generation of dislocations.
Because of such problems, change of the waveguide thickness is limited and, as a result, the laser wavelength cannot be changed sufficiently. Therefore, wide wavelength range required in the optical subscriber network cannot be obtained and the system expansibility is limited.
On the other hand, separately from the above techniques, a method for implementing the multi-wavelength laser array by using a sampled grating in xe2x80x9cMonolithically integrated multiwavelength sampled grating DBR lasers for dense WDM applicationsxe2x80x9d, S. L. Lee, IEEE J.Selected Topics on Quantum Electron, vol.6, No.1, pp.197-206, January 2000. In this structure, the sampled grating substitutes for the diffraction grating in a mirror area of a distributed Bragg reflector (DBR) laser and wavelengths are selected by current injection.
However, because a laser resonator is divided into 4 or 5 sections and current should be injected to the sections separately, procedure and device operation are very complex. Also, since device length is typically long, quantum efficiency or output optical characteristic is deteriorated compared with a typical DBF laser.
As described above, in the conventional techniques, there are problems in that complex and time consuming procedure is used to fabricate the DFB laser array required for the optical subscriber network or the fabricated laser array does not cover sufficient wavelength range.
Therefore, there is a need for a new structure or a method to overcome the shortages of the conventional techniques, for the light source for the WDM optical subscriber network.
Therefore, it is an object of the present invention to provide a structure and a method for manufacturing a multi-wavelength laser array easily and economically by using a sampled grating.
In accordance with an aspect of the present invention, there is provided a multi-wavelength semiconductor laser array comprising a substrate; a plurality of laser stripes arranged with a predetermined space on the substrate, each being divided into two sections; a multiplicity of asymmetric sampled gratings having different sampling periods from each other on the bottom of each active layer; and a number of effective refractive index changing layers, each arranged on one section of each laser stripe to make Bragg wavelengths different at the two sections.
Preferably, the asymmetric sampled grating has sampling periods to make the xe2x88x921-st order or +1-st order reflection wavelengths at both sections of the ASG laser cavity match with each other.
In accordance with another aspect of the present invention, there is provided a method for fabricating a multi-wavelength semiconductor laser array, comprising the steps of: (a) forming a multiplicity of diffraction gratings having the same period on a substrate; (b) forming asymmetric sampled gratings having sampling periods different from each other by periodically removing some parts of the diffraction gratings; (c) forming active layers on the asymmetric sampled gratings; (d) forming effective refractive index changing layers for one of the two sections.